In order that a plant is able to grow efficiently it is important that it is supplied with water and nutrients. Nutrients are often added to a plant growing medium, such as soil, in the form of fertilisers. Because it is difficult to direct fertilisers only around the roots of a plant, these nutrients are applied in excess in order to ensure sufficient supply to the roots to allow efficient take up. However, such excessive fertilisation may have environmental impacts, in particular under conditions of heavy rain nutrients may be washed from the soil to water courses. Such an excess of nutrients in a water course can lead to problems such as eutrophication, where plant growth in a water course is accelerated due to the excess of nutrients.
In an attempt to overcome this problem controlled release fertilisers have been developed in which a granule of one or more fertilisers is encapsulated within a layer of a compound which allows water to pass through it but restricts the movement of the fertiliser in solution. The rate of outflow of nutrient solution through the selectively permeable layer is typically controlled by temperature, the layer having pores therein which expand upon heating thereby allowing the nutrient solution to pass through. The controlled release fertiliser system suffers from the disadvantage that the nutrients passing through the selectively permeable layer are in a saturated solution. If the water of this solution evaporates it leaves the nutrient in a crystalline form making it unavailable to the plant. Furthermore, at times of excess water in the soil, the nutrients may well be leached away from the plant before they can be absorbed.
In locations where a lack of water is also a problem irrigation of plants may be undertaken. However, the regular supply of water for plants in locations where water is inherently scarce is expensive, wasteful of resources and potentially damaging to downstream environments. In such locations super absorbent polymer, capable of retaining large volumes of water, may be used. Such polymers absorb large quantities of water and can retain them against evaporation far more efficiently than soil alone. Where nutrient is also required the polymer may be loaded with such nutrients by soaking the polymer in a concentrated solution of a fertiliser and subsequently dehydrating it. This process is repeated in order to maximise the quantity of nutrient in the polymer structure. As a result, this is an expensive process and it is not possible to provide the super absorbent polymer with sufficient loading of nutrients to supply the long term needs of the plants. Furthermore, the super absorbent polymers also suffer from the disadvantage that the nutrient is easily lost and a large proportion of the fertiliser may be dumped from the super absorbent polymer upon the first significant rainfall.